The Binocular Blog - Part 5: Exit Pupil & Interpupillary distance

We are still not finished with the numbers game in the binocular blog. There are two other numbers which are also to be taken into account. One is the Exit Pupil and the other is the Interpupillary Distance (IPD).

EXIT PUPIL

The exit pupil is the circle of light seen when the binocular is held about a foot away from the eyes. The larger the exit pupil, the brighter the image will be, and so better viewing in low light. The larger the objective lens diameter the brighter the image but the reverse applies as the magnification increases.

 

The exit pupil becomes important in dim light situations, like at dawn, dusk or heavily overcast conditions. A minimum exit pupil of 5 mm will give a bright image in such conditions. However that depends on the size of our pupils. 

Normal pupils in human eyes vary  between 2 to 8 millimeters and is always constricting and dilating depending on the ambient light.  In broad daylight it will be smaller, between 2-4 millimeters but as the light falls the pupils dilate, almost up to 8 or 9mm in near darkness. This range is applicable for most young people. However,  as we grow older our pupils may not dilate well even in dim light. The thumb rule should be that the exit pupil of the binoculars should be slightly bigger than our pupils.

Exit pupil in the Vortex Viper HD 12x50 (top) versus the Zenith 7x50 (bottom)

It can be calculated by dividing the objective lens diameter by the magnification factor. Though the two binoculars in the photo, have the same objective lens diameter, the Zenith (7x50) will have a larger exit pupil compared to the Vortex Viper HD (12x50).  

COMPARISON OF BINOCULARS IN MY POSSESSION	Magnification	Objective Diameter	Exit Pupil
BUSHNELL H2O 10X25	10	25	2.5
BUSHNELL TROPHY 10X42	10	42	4.2
ZEISS TERRA ED 8X42	8	42	5.25
SUPER ZENITH 7X50	7	50	7.14
VORTEX VIPER HD 12X50	12	50	4.16

In dim light situations, a minimum exit pupil should be 5 mm, as mentioned earlier. Since pupils may not dilate well in older people a binocular with a large exit pupil will be ideal in such conditions. 

However,  in very bright condition the small pupil, say about 2 mm, will cut out most of the light coming to the eye from a binocular with an exit pupil larger than the pupil of the observer, and so, much of the brightness is 'wasted'. On the contrary, if the pupil is large, say 6 or 7 mm; a binocular with a smaller exit pupil, like 4 mm or less, will make the observed object darker. In older persons, whose pupil may not dilate much even in dim light, an binocular with an exit pupil of less than 5 mm will also produce a bright image as that will be sufficient to produce a column of light wider than the pupil. 

INTERPUPILLARY DISTANCE (IPD)

The other aspect that has to be taken into consideration is the IPD of the binocular. That is the distance between the center of the pupils of the your two eyes. Unless your binocular is aligned correctly to your pupils you will not be able to the full field afforded by the binoculars. 

IPD scale on the Zenith 7x50

 The hinge in the binocular serves to adjust your IPD before you attempt to focus the instrument. Most binoculars don't provide and IPD setting scale so you have to do it while looking through. Some older Porro prism binoculars have a scale so you can set you IPD before lifting it to your eyes. 

The IPD of an average adult will be between 55 to 75 mm Most binoculars have IPD range of around just under 60 mm to little over 70 mm which is more than adequate for normal people, especially at the wider end.

When opened fully the IPD of a compact binocular (10x25) is almost the same as a full size binocular (12x50)

 

IPD when fully open is around 75 mm.

However, there are people with very narrow set eyes, and they will find that most binoculars are unfriendly to them. For such people the of compact binoculars are very useful. 

In a double hinged compact binocular the IPD can be reduced further due to the hinge design.

Some of the compact binoculars are double hinged so that they can be folded into a very tiny package for storage. The barrels fold under the bridge, almost touching each other.

Full sized binocular compared to compact binocular with hinges fully closed in both.

Such binoculars can be used by people with very small IPD of less than 55 mm. In fact once folded completely the IPD is around 40 mm!

IPD of a double hinged binocular is just around 40 mm when closed fully.

This is another reason why it is always better to go out and try the pair you have set your mind on, especially if your eyes are very narrow or widely set.


For those who are impatient to go through the entire blog series, I'm giving below direct links to all the posts. You can go directly to the page you are interested in, though I suggest you go through each post as all of them contain some information that will be useful when you consider purchasing a binocular for your self.

1.       Introduction
2.       Part – 1: Anatomy of a Binocular
3.       Part – 2: Purpose and Portability
4.       Part – 3: Magnification, Objective diameter & FOV
5.       Part – 4: Eye Relief
6.       Part – 5: Exit pupil & Interpupillary distance
7.       Part – 6: Image brightness
8.       Part – 7: Focusing fundamentals
9.       Part – 8: Weather sealing
10.   Part – 9: Purchase decision
11.   Part – 10: Care & Storage
12.  Afterthoughts and accessories